The structure of oxytocin will be determined by use of direct methods. Two crystalline salts of oxytocin have been prepared and data collected. Starting phases for a tangent refinement procedure have already been obtained. Once the structure of oxytocin in this crystal form has been established, the question of oxytocin molecular flexibility will be investigated in the solid state by structure analyses of a crystalline non-isomorphous oxytocin salt, and of crystalline deamino-oxytocin. Rigid-body vector search techniques and direct methods will be employed. There are many oxytocin and vasopressin analogs and we shall attempt to obtain crystalline preparations of some of these to study conformational stability. The structure of the isolated ring will also be studied. If crystalline preparations of the neurophysin-oxytocin complex can be obtained, a structure study will be begun.